The Third Generation Partnership Project Long Term Evolution (3GPP LTE) is a mobile communication standard intended to boost the data rates in cellular networks. 3GPP LTE is also referred to simply as LTE, and is also known as Evolved Universal Terrestrial Radio Access (E-UTRA), an evolution of Universal Terrestrial Radio Access (UTRA). For the uplink of LTE, that is, for transmission from a user equipment (UE) to a base station, Discrete Fourier Transform spread orthogonal frequency division multiplexing (DFTS-OFDM) is used to reduce the peak-to-average power ratio of the signal. A UE is assigned a channel occupying a channel bandwidth, and within the channel bandwidth the UE transmits a DFTS-OFDM signal occupying a transmission bandwidth. The channel bandwidth is divided into resource blocks of equal spectral width, and the UE is allocated a variable number of the resource blocks, the number being updated once every millisecond to adapt to varying channel conditions and to the amount of data to be transmitted by the UE. Therefore, the transmission bandwidth can change each millisecond, depending on the number of resource blocks allocated to the UE. The spectral width of each resource block is 180 kHz, which corresponds to twelve sub-carriers of the DFTS-OFDM signal.
The relationship between resource blocks, transmission bandwidth and channel bandwidth is depicted in FIG. 1. Referring to FIG. 1, the resource blocks may be allocated to different UEs, and the resource blocks assigned to each UE are contiguous in the frequency domain. Therefore, different UEs may simultaneously have different transmission bandwidths. The transmission bandwidth configuration is the maximum number of resource blocks in the channel bandwidth which can be allocated to one or more UEs. The maximum transmission bandwidth is narrower than the channel bandwidth, there being a margin at each end to allow for roll-off of the spectrum of the signal transmitted by the UE.
The 3GPP technical specification TS 36.101, v10.3.0 defines several spectral limits on RF emissions from a UE outside of the assigned channel bandwidth, examples of which are illustrated in FIG. 2. More specifically, an Adjacent Channel Leakage power Ratio (ACLR) is illustrated for UTRA and for E-UTRA. In addition, a network may signal to a UE spurious emission limits, referred to as network signalled (NS) values. The network signalled values NS_03 and NS_07 are illustrated in FIG. 2. NS_07 is an especially stringent limit to meet, necessitating a very linear transmitter. For these examples the assigned channel bandwidth is 10 MHz, extending from 777 MHz to 787 MHz.
Therefore, there is a requirement for a transmitter that can meet demanding constraints on spectral emissions.